A flat panel display (which may also be referred to as a “liquid crystal display” (LCD), “flat panel”, and the like) is often used as the “screen” for mobile electronic products such as laptop computers and wireless handheld devices (e.g., cellular telephones, personal digital assistants (PDAs), etc.). A flat panel display typically comprises a matrix of liquid crystal elements that affect the optical contrast(s) presented to a viewer of the flat panel display. The optical contrast(s) are affected in response to one or more electronic signals that are applied to the liquid crystal elements.
FIGS. 1a through 1c show possible types of flat panels that are liquid crystal based. FIG. 1a shows a “transmissive” flat panel display 101a, FIG. 1b shows a “reflective” flat panel display 101b; and, FIG. 1c shows a “trans-reflective” flat panel display 101c. According to the “transmissive” flat panel display approach of FIG. 1a, electronic signals are directed to the liquid crystals that effectively modulate the amount of light emitted by the liquid crystals so as to present an overall image to a viewer of the flat panel display. Here, an “internal” light source (referred to as a backlight 104a) acts as a light source. The transparencies of the liquid crystals are individually modulated by the electronic signals such that the more transparent a liquid crystal becomes, the more light it emits from the perspective of a viewer of the flat panel display.
According to the “reflective” flat panel display approach of FIG. 1b, electronic signals are directed to the liquid crystals that effectively modulate their reflectivity. Here, an external light source 102b is the optical basis for forming an image. The modulated reflectivity of the flat panel 101b is able to help form an image by reflecting the optical energy from the external light source 102b at varying percentages over the surface of the flat panel in accordance with the modulating electronic signals.
The “trans-reflective” flat panel display of FIG. 1c combines the approaches observed in both FIGS. 1a and 1b. That is, electronic signals are used to modulate both the optical emission and the optical reflection of the liquid crystals in order to present an overall image to a viewer of the flat panel display. Regardless as to which type of flat panel display technology is used, a liquid crystal may be characterized in terms of its “transmittance”.
Here, higher transmittance corresponds to more light as observed by the viewer; and, lower transmittance corresponds to less light observed by the viewer. Thus, in the case of a “transmissive” display, higher transmittance corresponds to more light emitted by a liquid crystal (i.e., greater transparency); in the case of a “reflective” display, higher transmittance corresponds to greater liquid crystal reflectivity; and, in the case of a “trans-reflective” display, higher transmittance corresponds to more light emitted by a liquid crystal and greater liquid crystal reflectivity.
Flat panel displays are often classified as “active” or “passive”. An active flat panel display matrix typically includes a transistor coupled to each liquid crystal that “drives” its corresponding liquid crystal. A passive flat panel display matrix omits the aforementioned transistor. FIG. 2a shows an embodiment of a circuit model for an active flat panel display “dot”. A dot is the basic unit of transmission in a flat panel display; and, therefore, a dot includes a liquid crystal element. A pixel typically comprises three “dots”: one red dot, one green dot, and one blue dot. According to the circuit model of FIG. 2a, the liquid crystal dot is represented as a capacitor “C”. The transistor “Q”, as is consistent with the aforementioned description of an active flat panel display, is configured to drive the liquid crystal C.
As flat panel displays are usually organized into a matrix having rows and columns, one transistor node is coupled to a device that drives the row to which the dot circuit belongs; and, another transistor node is coupled to a device that drives the column to which the dot circuit belongs. In the dot circuitry example of FIG. 2a, the row driver is coupled to the transistor's gate node 212 and the column driver is coupled to the transistor's drain node 211.
The transistor Q is turned “on” or “off” in response to the row node 212 voltage. When the row node 212 voltage is sufficient to turn the transistor Q “on”, the transistor Q effectively acts as a short circuit. This allows the voltage applied at the column node 211 to appear at the capacitor C electrode that is opposite the common node 213. Hence, the voltage across the capacitor Vc is approximately equal to the difference between the column node 211 voltage and the common node 213 voltage.
The transmittance of the liquid crystal C depends upon the root-mean-square (rms) of the voltage Vc that is applied across the liquid crystal. FIG. 2b shows some exemplary waveforms that are consistent with present day applications. Firstly, a common voltage 203 (that is applied at common node 213 of FIG. 2a) alternates between a pair of voltages. Although not a strict requirement, the common voltage is often made to alternate between a positive voltage (e.g., +7 v) and a negative voltage (e.g., −2 v). Hence, the common voltage 203 is often regarded as alternating over time between a pair of phases: 1) a positive phase where the common voltage is positive (one of which is shown as being over time period 210 in FIG. 2b); and 2) a negative phase where the common voltage is negative (one of which is shown as being over time period 211 in FIG. 2b).
The column voltage 201 is crafted, when the transistor is “on”, so as to create a specific rms voltage across the capacitor C (so that a specific transmittance is associated with the liquid crystal C) in light of the alternating common voltage 203. For example, as seen in FIG. 2b, during the positive phase(s), the column voltage is +2 v; and, during the negative phase(s), the column voltage is +3 v. Thus, also as seen in FIG. 2b, during the positive phase(s) Vc=−5 v (i.e., 2 v−7 v=−5 v); and, during the negative phase(s) Vc=+5 v (i.e., 3 v−(−2 v)=+5 v). As such, for the exemplary embodiment of FIG. 2b, the rms voltage for the voltage across the capacitor is 5 v. As mentioned above, the transmittance of the liquid crystal C depends upon the root-mean-square (rms) of the voltage that is applied across it. FIG. 3 shows an exemplary depiction 301 of the transmittance as a function of the applied rms voltage.